1. Field of the Invention
The invention relates to a turbocharged internal combustion engine, in particular a turbocharged internal combustion engine with at least one high-pressure and one downstream low-pressure stage, whose turbines are designed as single-flow or double-flow type, as well as with piping that connects the high pressure turbine on the inlet side to the exhaust side of the engine and the outlet side to the low-pressure turbine with at least one bypass channel lockable by means of a pipe switch connecting the exhaust side of the engine on the inlet side to the low-pressure turbine.
2. Description of the Related Art
On such an internal combustion engine with two-stage turbocharging disclosed in DE 195 14 572 A1, a high-pressure stage and a low-pressure stage are arranged in series in a turbocharger in the lower speed range of the internal combustion engine. The exhaust initially flows through the high-pressure turbine and then through the low-pressure turbine. The turbocharging air is first compressed by the low-pressure compressor and then by the high-pressure compressor and fed, after cooling in a heat exchanger, to the fresh-air side of the internal combustion engine. As the rotational speed of the internal combustion engine increases, a changeover can be made to single-stage compression exclusively in the low-pressure compressor in that the high-pressure turbine is completely bypassed by means of an exhaust-side pipe switch and, appropriately, the high-pressure compressor can be fully bypassed via a turbocharging-air-side pipe switch.
A disadvantage of such changeover turbocharging can be seen by the fact that in the event of frequently desired load and speed changes of the internal combustion engine, very often a changeover must be made between one-stage and two-stage mode of operation of the turbocharging unit.
Consequently, there may be a loss of traveling comfort, i.e. unsteady acceleration and braking power response.
A further internal combustion engine according to the preamble is disclosed in DE 39 03 563 C1. Here, too, a changeover is provided from two-stage to one-stage turbocharging. The changeover is affected by means of a pipe switch arranged between the outlet side and the high-pressure turbine. Thus losses in traveling comfort may occur here also.
The same problem is with DE 25 44 471 A1 which features an exhaust gas recirculation (EGR), because the pipe switch is arranged between the outlet side and the high-pressure turbine. A different type of EGR is disclosed in the U.S. Pat. No. 5,142,866, wherein the bypass is located downstream of the high pressure turbine.
The invention is based on the problem of providing an internal combustion engine which responds to fast load and speed changes without unsteady acceleration and braking power response. The turbocharging pressure is intended, in the case of accelerationxe2x80x94i.e. when the vehicle is to be acceleratedxe2x80x94to build up rapidly and be capable of being adapted infinitely and variably to the engine requirements.
This problem is solved by the characteristics described below.
Through the characteristics according to the invention, specifically the following is achieved:
Because there is a continuously flow through the high-pressure turbine at least to a certain extent, and this flow circulates, it is ensured that in the case of acceleration a minimum turbocharging pressure exists and, in particular, the rotational speed of the HP rotor is at a favorable initial level. Furthermore, the individual exhaust mass flows can be supplied to the high-pressure turbine, the low-pressure turbine, or the fresh air side by the arrangements according to the invention with the help of the central processing unit and the pipe switch to the extent desired in each case, so that an optimization of the mode of operation of the engine can be made with respect to minimum fuel consumption and/or minimum pollutant emission.
With corresponding load and increasing rotational speed of the engine, a fast response of the high-pressure turbine is thus ensured in that the expansion work is shifted in the direction of the high-pressure turbine, i.e. through extensive closure of the bypass channel by means of a pipe switch, the largest portion of the exhaust flow is fed to the high-pressure turbine. If, at low load and with small exhaust mass flows, a low-consumption, small load and, above all, exhaust counter pressures are desired in this operating range, the expansion work of the exhaust can for the greater part take place in the low-pressure turbine and possibly by appropriate positioning of the pipe switch via the exhaust return, independently of the rotational speed of the engine by opening the bypass channel.